U.S. Geological Survey and The National Academies; USGS OF-2007-1047, Short Research Paper 047; doi:10.3133/of2007-1047.srp047

Geophysical survey reveals tectonic structures in the Amundsen embayment, K. Gohl,1 D. Teterin,2 G. Eagles,1 G. Netzeband,3 J. W. G. Grobys,1 N. Parsiegla,1 P. Schlüter,1 1 1 2 V. Leinweber, R. D. Larter,r G. Uenzelmann-Neben, and G. B. Udintsev 1Alfred Wegener Institute for Polar and Marine Research, Postbox 120161, 27515 Bremerhaven, Germany ([email protected]) 2Vernadsky Institute of Geochemistry and Analytical Chemistry, Laboratory of Geomorphology and Tectonics of the Floor, 19 Kosygin St., 117975 Moscow, Russia ([email protected]) 3Institute for Geophysics, University of Hamburg, Bundesstr. 55, 20146 Hamburg, Germany (now at: [email protected]) 5British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, UK ([email protected]) Abstract The embayment of West Antarctica is in a prominent location for a series of tectonic and magmatic events from Paleozoic to Cenozoic times. Seismic, magnetic and gravity data from the embayment and Pine Island Bay (PIB) reveal the crustal thickness and some tectonic features. The Moho is 24-22 km deep on the shelf. NE- SW trending magnetic and gravity anomalies and the thin crust indicate a former rift zone that was active during or in the run-up to breakup between and West Antarctica before or at 90 Ma. NW-SE trending gravity and magnetic anomalies, following a prolongation of Peacock Sound, indicate the extensional southern boundary to the Bellingshausen Plate which was active between 79 and 61 Ma. Citation: Gohl, K., D. Teterin, G. Eagles, G. Netzeband, J. W. G. Grobys, N. Parsiegla, P. Schlüter, V. Leinweber, R. D. Larter, G. Uenzelmann- Neben, and G. B. Udintsev (2007), Geophysical survey reveals tectonic structures in the Amundsen Sea embayment, West Antarctica, in Antarctica: A Keystone in a Changing World – Online Proceedings of the 10th ISAES, edited by A. K. Cooper and C. R. Raymond et al., USGS Open-File Report 2007-1047, Short Research Paper 047, 4 p.; doi:10.3133/of2007-1047.srp047

Introduction The newest geophysical data from the Amundsen Sea The Amundsen Sea embayment with its eastern sector, Embayment and PIB (Figure 1) reveal crustal thickness Pine Island Bay (PIB) (Figure 1), is in a tectonically and tectonic features and lineations. These early results prominent position suggested to be the location of a are discussed in context with plate-kinematic crustal boundary between the Marie Byrd Land block to reconstructions of the early stages of continental breakup the west and the / blocks to and southern Pacific seafloor spreading. the east which was active during the Mesozoic break-up of Gondwana or even before (e.g. Dalziel and Elliot, 1982; Storey, 1991; Grunow et al., 1991). Eagles et al. (2004) illustrate that early Pacific-Antarctic separation evolved first as rifting and crustal extension along the present-day between Chatham Rise and (Grobys et al., 2007). It possibly continued along the present Great South Basin between the Campbell Plateau and the of at 90 Ma until the rift was abandoned in favour of a new extensional locus to the south, forming the earliest oceanic crust between Campbell Plateau and Marie Byrd Land at 84-83 Ma. The boundary between Chatham Rise and Campbell Plateau – with Bounty Trough closed before 90 Ma – is situated off the western Figure 1. Overview map of the Amundsen Sea Amundsen Sea Embayment at about 120°-125° W. embayment and Pine Island Bay showing the satellite- derived gravity anomaly grid of McAdoo and Laxon The Bellingshausen Plate (Stock and Molnar, 1987) (1997) and the locations of two deep crustal seismic moved independently on the southern flank of the mid- (dashed red lines) and multi-channel seismic profiles (red Pacific spreading ridge from Late Cretaceous times until lines) collected during RV Polarstern expedition ANT- about 61 Ma when a major plate reorganisation occurred XXIII/4. The dotted bold black lines mark the in the South Pacific (e.g. Larter et al., 2002; Eagles et al., approximate location of the interpreted incipient narrow 2004). The small plate’s western boundary was situated in rift basin. Its trend is aligned with a gravity anomaly north the area of the Marie Byrd Seamounts (Heinemann et al., of Thurston Island. The dashed black lines illustrate the 1999), its eastern transpressional boundary along a gravity presumed location of the southern Bellingshausen Plate anomaly lineament in the western Bellingshausen Sea. boundary. The box marks the area of the Moho depth map The southern boundary may have been active along the of Figure 4. PT Pine Island Through, PGA Peacock WNW-ESE striking Peacock Gravity Anomaly (PGA) Gravity Anomaly, PS Peacock Sound, PG Pine Island across the outer continental shelf of the Amundsen Sea Glacier, TG Thwaites Glacier. embayment (Larter et al., 2002; Eagles et al., 2004). 10th International Symposium on Antarctic Sciences

Geophysical data acquisition in the Amundsen reveals a NE-SW trending system of near-parallel positive Sea Embayment magnetic anomaly lineations as the dominant feature in The Alfred Wegener Institute and British Antarctic the western embayment (Figure 2). The lineations run Survey cooperated closely during early 2006 with RV parallel to the trend of the satellite-derived gravity Polarstern’s expedition ANT-XXIII/4 and RRS James C. anomalies across the middle shelf and that of sharp Ross’ cruise JR141. Extensive geophysical datasets were positive linear gravity anomaly north of Thurston Island. collected as part of a research program dedicated to the The magnetic grid over the eastern PIB reveals scattered study of tectonic, sedimentary and glacial advance/retreat patches of positive and negative anomalies with no processes in the Amundsen Sea Embayment and PIB significant lineations, except for a the short WNW trend (Larter et al., 2007). For this paper, we primarily use the of positive anomalies west of the Peacock Sound. This resulting deep crustal seismic profiles, shipborne gravity anomaly lies on top of, and has the same orientation as, and magnetic data collected from the Polarstern, and an the PGA, but it does not follow the PGA farther extensive aeromagnetic survey of 20,000 km flown by the northwest. However, gravity modeling indicates an Polarstern helicopters (Figures 1 and 2). Due to an area elongated body of high-density, possibly magmatic of inaccessibility, the magnetic data mainly cover two material along the PGA. Some of the scattered magnetic separate areas: the western Amundsen Sea embayment anomalies in the south may be attributed to the numerous and the southern and eastern PIB . The analysis of igneous and meta-igneous bodies observed in outcrops on these data was complemented by the few previously- islands in PIB and along its eastern mainland shore as existing seismic and potential field data from the shelf observed by the SPRITE Group (1992) and on our break to the oceanic parts of the Amundsen Sea (Gohl et Polarstern cruise. al., 1997; Nitsche et al., 2000) and by the satellite-derived gravity anomaly field of McAdoo & Laxon (1997). We acquired deep crustal seismic wide-angle reflection and refraction data with one profile across the inner to middle continental shelf of the western embayment (AWI-20060100) and another running from the foot of the slope towards the Marie Byrd Seamount cluster of the continental rise (AWI-20060200) (Figure 1). Shots from an airgun cluster, consisting of 8 G.Guns™ with a total volume of 68 liters (4160 in3), fired with 190 bar at a 1-minute interval (150 m nominal spacing), resulted in good quality data with identified phases from the upper to lower crust and the uppermost mantle on both profiles. Restricted by massive pack-ice to the north, west and east and by drifting sea-ice patches, only nine ocean- bottom hydrophone (OBH) systems could be deployed at 18-19 km spacing on the shelf, in order to minimize the risk of instrument loss. Seven OBH systems with the same nominal spacing were deployed along the deep-sea Figure 2. Map of gridded magnetic anomalies in the profile. About 2200 km of multi-channel seismic (MCS) Amundsen Sea embayment from helicopter-magnetic profiles were acquired in the Amundsen Sea Embayment surveying and ship-board magnetic data of ANT-XXIII/4. and in PIB using a 600-m long analogue streamer. We Dashed black lines indicate linear anomaly trends. Thin also recorded low-fold normal-incidence reflection data dark blue lines denote seismic reflection profiles of ANT- from the airgun shots along both OBH profiles with the XXIII/4. Thick dark blue lines mark the locations of deep same streamer. 530 km of single-channel seismic data crustal seismic profiles (the southern profile AWI- were recorded in the embayment during the JR141 cruise 20060100 corresponds to model in Figure 3). (Larter et al., 2007). For this study, we use parts of the MCS dataset to identify the depth of the acoustic The P-wave phases recorded by eight OBH systems of basement to constrain thickness and geometries of the shelf profile AWI-20060100 were modeled for a crustal sediment cover in the seismic refraction and gravity velocity-depth structure using a raytracing and travel-time modelling process. Details of the seismic reflection data inversion method. The final velocity-depth model with regard to glacial-marine sedimentation processes will (Figure 3) shows a 22-23 km thick crust with velocities of be given elsewhere (e.g. Uenzelmann-Neben et al., this 2.0-3.5 km/s for the sediments above the acoustic volume). basement, 5.0-6.2 km/s for the upper crust below the Magnetic grid, seismic and gravity models acoustic basement, and 6.4-6.9 km/s for the lower crust. The combined helicopter and shipboard magnetic grid Pn phases indicate an upper mantle velocity of 7.9- Gohl et al.: Geophysical survey reveals tectonic structures in Amundsen Sea embayment, West Antarctica

8.0 km/s. The relatively thin crust, the regular vertical thinning process. These three observations lead to a velocity distribution, and the average crustal velocity of model of a failed initial rift or distributed extension in the about 6.4 km/s, all suggest that the inner continental shelf Amundsen Sea embayment crust. This rift must have been of the Amundsen Sea embayment rests on stretched active before 90 Ma or it accompanied the rifting in continental crust, but is largely unaffected by magmatic Bounty Trough and its northward translation of Chatham intrusions. Unfortunately, the coverage at the Rise at this time. northwestern end of the profile does not reach into the We interpret the WNW trending PGA and its area of the linear SW-NE trending magnetic anomalies. underlying high-density body as a magmatic zone which overprinted the NE-SW trending rift structure. This seems to confirm the suggestion by Eagles et al. (2004) that this zone acted as the southern boundary of the Bellingshausen Plate between 79 and 61 Ma. Plate- kinematics demonstrates that this plate boundary was active as a zone of relatively minor extensional and translational movements. Our new data show that magmatic intrusions were emplaced in some places along this boundary.

Figure 3. Velocity-depth model of OBH profile AWI- 20060100 across the inner and middle continental shelf of the western Amundsen Sea embayment. OBH stations are marked by triangles. Layers outside the coloured grid are not constrained by travel-time information but constrained by gravity modelling.

A series of 2-D gravity models were calculated along the seismic and other quasi-linear profiles across the Amundsen Sea embayment primarily using shipboard data complemented by satellite-derived gravity data (McAdoo and Laxon, 1997) in gaps and to extend the profiles. We constrained the models with observations of sediment thickness using seismic reflection data and the crustal structure and thickness from the two OBH profiles. Next, we integrated the 2-D models into a 3-D Figure 4. Moho depth map of the Amundsen Sea gravity anomaly model of the embayment. The resulting embayment derived from 2D (along profiles annotated map of Moho depth (Figure 4) shows the crust thinning with black lines) and 3D gravity anomaly modeling above a Moho at about 24 km depth beneath the eastern constrained by satellite-derived gravity data (McAdoo and embayment and about 22 km in the western embayment. Laxon, 1997) and seismic data (red lines). Beneath the oceanic crust, the Moho depth increases from 14-15 km in the east to 15-17 km in the west between the The presumed Paleozoic crustal boundary between the Marie Byrd Seamounts and the foot of the shelf slope. Thurston Island/Ellsworth Land block and the Marie Byrd Land block in PIB is not obvious from the potential field Tectonic implications data. If present, the difficulty in interpreting its presence The potential field data indicate two dominant and may be traced back to overprinting by Mesozoic and crossing trends of linear anomalies in the Amundsen Sea Cenozoic magmatic intrusions and volcanism (e.g. embayment. The linear gravity and magnetic anomalies of LeMasurier, 1990). What is more, the deeply incised the western Amundsen Sea embayment, running sub- inner and middle shelf of PIB with glacial troughs and parallel to each other, can be interpreted as indicating an channels reaching 1000-1500 m depth (Lowe and intrusive crustal origin. Their NE-SW trend parallels the Anderson, 2002; Larter et al., 2007) may obscure initial spreading center’s azimuth between Chatham Rise interpretations of the magnetic anomaly field. We agree and West Antarctica and can thus be related to processes with an earlier suggestion by the SPRITE Group (1992) occurring during breakup or just beforehand. The that the main deep glacial erosional trough, here named anomalously thin crust beneath the inner shelf - in Pine Island Trough (Figure 1), stretching from the mouth particular of the western embayment - suggests a crustal of the Pine Island Glacier to the middle shelf in NW and 10th International Symposium on Antarctic Earth Sciences

NNW orientation may have exploited such a former Grobys, J. W. G., K. Gohl, B. Davy, G. Uenzelmann-Neben, T. Deen, crustal block boundary. and D. Barker (2007), Is the Bounty Trough, off eastern New Zealand, an aborted rift?, J. Geophys. Res., 112, B03103, Conclusions doi:10.1029/2005JB004229. Grunow, A. M., D. V. Kent, and I. W. D. Dalziel (1991), New Seismic, magnetic and gravity surveying of the paleomagnetic data from Thurston Island: Implications for the Amundsen Sea embayment and PIB reveal important tectonics of West Antarctica and opening, J. Geophys. phases of the tectonic evolution in this area. The main Res, 96, B11, 17935-17954. Heinemann, J., J. Stock, K. Clayton, S. Hafner, S. Cande, and C. results are: Raymond (1999), Constraints on the proposed Marie Byrd Land– (1) The Moho depth of the inner and middle shelf is Bellingshausen plate boundary from seismic reflection data, J. between 24 and 22 km and thins from east to west. Geophys. Res., 104, 25321–25330. Larter, R. D., A. P. Cunningham, P. F. Barker, K. Gohl, and F. O. (2) Dominant NE-SW trending magnetic and gravity Nitsche (2002), Tectonic evolution of the Pacific margin of Antarctica anomalies and the thin crust indicate a former rift zone – 1. Late Cretaceous tectonic reconstructions, J. Geoph. Res., proposed to be active during or prior to the breakup 107(B12), 2345, doi:10.1029/2000JB000052. Larter, R.D., K. Gohl, C. D. Hillenbrand, G. Kuhn, T. J. Deen, R. between Chatham Rise and West Antarctica before or at Dietrich, G. Eagles, J. S. Johnson, R. A. Livermore, F. O. Nitsche, C. 90 Ma. J. Pudsey, H.-W. Schenke, J. A. Smith, G. Udintsev, and G. (3) NW-SE trending gravity and short magnetic Uenzelmann-Neben (2007), West Antarctic Ice Sheet change since the last glacial period, EOS Trans., Am. Geophys. Union, 88, 189-196. anomalies in the prolongation of the Peacock Sound are LeMasurier, W. E. (1990), Late Cenozoic volcanism on the Antarctic consistent with the proposed tectonic activity and location plate: an overview, in Volcanoes of the Antarctic Plate and Southern of the southern Bellingshausen Plate boundary between , edited by W. E. LeMasurier and J. W. Thomson, Am. 79 and 61 Ma. Geophys. Union Antarct. Res. Ser., 48, pp. 1-19. Lowe, A. L., and J. B. Anderson (2000), Reconstruction of the West (4) A presumed crustal block boundary between Thurston Antarctic ice sheet in Pine Island Bay during the Last Glacial Island/Ellsworth Land block and the Marie Byrd Land Maximum and its subsequent retreat history, Quat. Sci. Reviews, 21, block cannot be clearly observed in PIB. It is possible, 1879-1897. McAdoo, D.C., and S. Laxon (1997), Antarctic tectonics: Constraints however, that the glacial Pine Island Trough exploited from an ERS-1 satellite marine gravity field, Science, 276, 556-560. such a former crustal block boundary or inherited Nitsche, F. O., A. P. Cunningham, R. D. Larter, and K. Gohl (2000), structure. Geometry and development of glacial continental margin depositional systems in the Bellingshausen Sea, Marine Geology, 162, 277-302. Acknowledgments. We gratefully acknowledge the excellent support of SPRITE Group (1992), The southern rim of the : Captain Uwe Pahl and his officers and crew of RV Polarstern during Preliminary geologic report of the Amundsen Sea - Bellingshausen expedition ANT-XXIII/4. We thank Ernst Flüh of IfM-GEOMAR for Sea cruise of the Polar Sea, 12 February – 21 March 1992, Ant. J. of lending us four OBH systems. Many thanks go to Ian Dalziel and the U.S., 27, 11-14. Michael Studinger for very helpful reviews and to the co-editor Bryan Stock, J., and P. M. Molnar (1987), Revised history of early Tertiary Storey for the editorial work. plate motion in the south-west Pacific, Nature, 325, 495-499. Storey, B. C. 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